(1) Field of the Invention
The present invention relates to rolling-traction variators of the type in which drive is transmitted from one race to another by at least one roller whose orientation is variable in accordance with changes in variator drive ratio. More specifically it concerns such a variator in which the roller is mounted upon a carrier gear which is controlled through sun and ring gears.
(2) Description of Related Art
The word “variator” is used herein to refer to a device which transmits rotary drive from a rotary input to a rotary output at a continuously variable variator drive ratio (the ratio of the input's speed to the output's speed). Variators are particularly, but not exclusively, applicable in motor vehicle transmission applications. One known form of rolling-traction type variator uses at least two co-axially mounted races having opposed faces which are shaped so that the races together form an approximately toroidal space. At least one roller is positioned in the space between the races and runs upon their shaped faces to transmit drive from one race to the other. Changes in the inclination of the roller are associated with changes in the relative speeds of the races, and hence in the variator drive ratio.
Some mechanism is needed to control roller inclination, and the prior art contains numerous examples. Typically such mechanisms do not act by directly applying a torque to the roller's mountings to tilt the roller. Instead, the roller is mounted in such a manner that displacing it causes it to steer itself, due to the forces exerted on it by the races, to a new inclination. The steering effect arises because the roller seeks a position in which its own axis coincides with the common axis of the variator races, since in any other condition the motion of the roller is non-parallel to that of the races in the area where they engage with each other. The control mechanism serves to regulate the roller's displacement.
Examples of such mechanisms are found in the applicants' prior published patent cases including PCT/GB03/00259 (WO 03/062670) and JP2006-292079A. In many examples, the displacement needed to cause the roller to steer itself is along the circumferential direction (about the common axis of the variator races) and, by allowing the rollers to tilt about an axis which is inclined to the radial plane, a relationship is established between roller displacement and roller inclination. An actuator is provided for urging the roller along the circumferential direction and so influencing its displacement, and correspondingly influencing the variator ratio.
The principles are illustrated in FIGS. 1 to 3, which are highly simplified representations of a variator viewed along the direction of the races' common axis (FIGS. 1b, 2b and 3b) and along a direction radial to said axis (FIGS. 1a, 2a, 3a). The variator's input and output races 111, 112 are represented in the radial views by straight lines, although in a real variator they are, as noted above, shaped to form a substantially toroidal cavity, and in the axial views they are seen to be circular. Roller 113 (which is one of a set, although the others are omitted from the drawings for the sake of simplicity) is arranged between the input and output races 111, 112, which are urged toward one another to provide traction between the roller and the races. The roller is mounted in a carriage 116 in a manner which permits the roller to rotate freely about its own axis P. The carriage 116 is connected to a piston of a hydraulic actuator 115. Power can in principle flow in either direction through the variator—from input to output or vice versa. Consider the case in which power flows from input to output. In this case the input race 111 turns the roller 113 (its direction of rotation is indicated as w1 in the drawings) and the roller 113 drives the output race 112 (whose direction of rotation is shown as w3). A traction force F11 is applied to the roller 113 by the input race 111, which is driving the roller 113. A traction force F12 is applied to the roller by the output disc, which is driven by the roller 113. The sum of the traction forces F11+F12 is reacted through the hydraulic actuator 115 and must be balanced by the actuator's force.
A momentary imbalance between the traction forces F11+F12 and the force of the actuator 115 causes the roller 113 to move. Suppose for example that, starting from a condition in which the variator is in equilibrium, the force applied by the actuator 115 is reduced. The traction force F11+F12 will then momentarily dominate, and the roller will move toward the actuator 115, as seen in FIG. 2. Referring to FIG. 2b, if the velocity vector V′r of the roller 113 at the region 117 where it engages the output race 112 were to remain unchanged, it would then be non-parallel to the velocity vector V′d of the surface of the output race 112 in the same region. The result is a traction force vector F14 acting on the roller 113 tending to cause it to tilt. A similar action between the input race 111 and the roller 113 produces a traction force on the roller at its region of contact (not shown) with the input race, and the two forces on the roller 113 form a couple, producing a steering effect upon the roller 113. That is, the roller 113 is caused to tilt about a steering axis 119. Note that this steering axis 119 forms a castor angle B to the radial plane 118. As a result of this castor angle, the tilting motion of the roller 113 is able to restore parallelism of the vectors V′r and V′d. The roller thus tilts (as in FIG. 3b) until it reaches a position in which the said vectors are parallel, and the aforesaid couple is thus reduced to zero. As the roller moves back and forth, its tilt (and correspondingly the variator's drive ratio) varies accordingly.
The total torque that must be reacted from the rollers to the variator's housing is often referred to as the “reaction torque”, and is equal to the sum of the torques upon the variator's input race 111 and output race 112. Note that this torque can only be reacted through the hydraulic actuators 115. Hence by regulating the hydraulic fluid pressures in the actuators 115, the reaction torque is directly regulated. The rollers automatically move to positions which cause the variator to generate a reaction torque corresponding to the said fluid pressures. It is thus the reaction torque that is directly regulated, and not the variator's actual drive ratio. This mode of control is thus sometimes referred to as “torque control”.
A different arrangement for controlling the variator rollers is described in Torotrak (Development) Limited's published International patent applications WO2007/065900 and WO2005/121602, both of which disclose variators in which each of a set of rollers is carried upon a respective carrier gear which meshes with a radially inner sun gear and a radially outer ring gear in the manner of a planetary gear in an epicyclic gear train. In this type of arrangement, rotation of the sun gear relative to the ring gear causes the carrier gear to turn, and it is this turning of the carrier gear that causes the steering effect upon the roller needed to control the roller's tilt.
In this type of arrangement it is necessary to control the motion of the sun and ring gears in order to control the variator, and the aforementioned International patent applications contain various mechanisms for this purpose. Among these are arrangements in which one or a set of control pinions is provided which, like the carrier gears, mesh with the sun and ring. In particular, WO2007/065900 discloses an arrangement in which a control pinion (referred to therein as a planet) is coupled to a hydraulic actuator in a manner which enables the actuator to move the pinion back and forth, but prevents the pinion from rotating. In this way the actuator controls the positions of both sun and ring gears.
All the arrangements in this prior art document permit the control pinion(s) and the carrier gears to move back and forth about the common axis of the variator races, with the forces on the control pinions being reacted through, and hence controlled by, some form of actuator. In this way the reaction torque can be regulated.